For some reason...

Those are the threads you ignored the last time you visited. I usually click "Mark forums read" when I finish, though, so I usually don't have that section -- only if I was doing admin stuff or a thread caused me to spend a lot of time doing something else.

I developed the habit of marking forums read when I finished back in the day, but I think it changed one day and I broke the habit. Now I don't bother.